Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the electrical current. Inorganic light-emitting diodes (LEDs) are also used in displays.
Most flat-panel displays use active-matrix thin-film transistors formed in a thin semiconductor layer on a substrate to control an array of light-emitting or light-controlling elements. However, the thin layer of semiconductor material is typically amorphous or has localized crystals, for example in amorphous silicon or polysilicon. These semiconductor material structures have a much lower performance than crystalline semiconductors found in typical integrated circuits. Moreover, it is difficult and expensive to make thin-film semiconductor structures on large substrates, limiting the size of integrated flat-panel devices such as displays. Passive-matrix control structures that do not require a thin-film semiconductor layer are also known but are limited in size resolution, and refresh rate.
At least in part because of these technical challenges, flat-panel displays were originally relatively small, for example having a display substrate diagonal measurement of only a few centimeters. As display substrate processing technology has improved, displays have increased in size, for example displays over three meters in diagonal have been demonstrated.
Large-format outdoor displays typically use inorganic light-emitting diodes (LEDs) individually mounted in a frame. In some displays, groups of LEDs are mounted in tiles and the tiles are assembled into the final display. If an LED in a tile fails, the faulty tile is removed and a good tile replaces the faulty tile. Moreover, tiles can be tested before assembly, increasing display yields. The use of tiles increases the available size of a display since each tile is separately made and is much smaller than the size of the display itself. However, it is in general challenging to provide electrical interconnections to the pixels in the tiles and to maintain a constant pixel pitch across the tile boundaries.
A variety of tiled display structures are known. U.S. Pat. No. 5,563,470 discloses a tiled panel display assembly with a common substrate on which a plurality of small display tiles are mounted in an array and electrically interconnected to form a large-area panel. Each tile includes a plurality of contact pads that are aligned with corresponding contact pads on the substrate. Solder joints between corresponding contact pads mechanically align and secure the tiles on the substrate and provide electrical connections therebetween. Selected substrate contact pads are electrically interconnected to provide electrical connections between adjacent tiles. Each of the tiles contains a plurality of metal-filled vias that connect contact pads on the under surfaces of the tiles to electrodes on the upper surface of the tile. Alternatively, electrical connections extend around the outer peripheral surface of the tile substrate. U.S. Pat. No. 8,531,642 shows a similar wrap-around electrical connection.
EP1548571 describes a configurable and scalable tiled OLED display. The OLED materials are deposited in a passive-matrix configuration on a transparent substrate and then interconnected with solder bump technology to a printed circuit board located on top of the transparent substrate. U.S. Pat. No. 6,897,855 describes a different tiled OLED structure with display tiles having picture element (pixel) positions defined up to the edge of the tiles. Each pixel position has an organic light-emitting diode (OLED) active area that occupies approximately 25 percent of the pixel area. Each tile includes a memory that stores display data, and pixel-driving circuitry that controls the scanning and illumination of the pixels on the tile. The pixel driving circuitry is located on the back side of the tile and connections to pixel electrodes on the front side of the tile are made by vias which pass through portions of selected ones of the pixel areas that are not occupied by the active pixel material. U.S. Pat. No. 6,897,855 also describes a tiled structure that employs vias through substrates to provide the electrical connections from the driving circuitry to the pixels on the display tiles, as does U.S. Pat. No. 6,853,411. U.S. Pat. No. 6,853,411 also describes locating pixel-driving circuitry beneath an OLED light emitter. Such a structure requires additional layers in a tile structure.
In an alternative arrangement, U.S. Pat. No. 7,394,194 describes a tiled OLED structure with electrical standoffs connecting OLED electrodes on a tile substrate with conductors on a back panel. The electrical standoffs are located on the edge of each tile to avoid compromising the environmental integrity of the OLED materials on the tile.
Inorganic light-emitting diode displays using micro-LEDs (for example having an area less than 100 microns square or having an area small enough that it is not visible to an unaided observer of the display at a designed viewing distance) are also known. For example, U.S. Pat. No. 8,722,458 entitled Optical Systems Fabricated by Printing-Based Assembly teaches transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate. U.S. Pat. No. 5,739,800 describes an LED display chip with an array of micro-LEDs mounted on a mounting substrate and electrically connected to a driver substrate. However, this arrangement requires multiple substrates and the use of vias to connect integrated circuits on the driver substrate to the LED display substrate and is not suitable for a scalable tiled structure.
Some displays use light-emitting structures on a backplane together with integrated circuits mounted on the backplane to provide control signals to the light-emitting structures. As discussed in U.S. Pat. No. 5,686,790, integrated circuits mounted on the light-emitting side of the backplane unnecessarily increase the size of the backplane while integrated circuits mounted on the side of the backplane opposite the light-emitting structures require electrical vias through the backplane or electrical leads wrapped around the edge of the backplane to electrically connect the integrated circuits with the light-emitting structures. Such vias and leads are difficult and expensive to construct. Integrated circuits located within the display area of a display reduce the resolution and aperture ratio of the display. In flat-panel displays such as LCDs and OLEDs, a reduced aperture ratio also reduces the brightness or lifetime of the display.
Multi-layer printed circuit boards (PCBs) are widely used in digital electronic systems to interconnect electronic elements such as integrated circuits and passive components such as resistors and capacitors. Such printed circuit boards include layers of insulating material interdigitated with patterned conductive layers such as etched copper sheets laminated with electrically conductive through-hole vias to interconnect the electronic elements, for example as disclosed in U.S. Pat. No. 4,591,659. However, these PCBs can be limited in the spatial resolution provided for integrated circuit electrical interconnections. Daughter cards used in conjunction with motherboards (i.e., smaller printed circuit boards mounted upon and electrically connected to larger printed circuit boards) are also known but likewise have limited spatial resolution, orientation, and integration provided for integrated circuits.
Thus there remains a need for a display tile structure that is simple to make and has increased brightness or lifetime in a robust structure for a tiled display.